Various topologies for inverters for photovoltaic systems are known from the Journal Bodo's Power Systems, Issue August 2008, pages 20 to 24, Michael Frisch and Temesi Ernö, Vincotech: “High Efficient Topologies for Next Generation Solar Inverter”. An inverter comprising two half-bridges connecting the two input lines and each half-bridge comprising two pulse-operated switches with reverse connected diodes belongs to these topologies as depicted in FIG. 13 of the publication. In the inverter, the switches arranged between the center points of the half-bridges and one of the input lines are IGBT semiconductor switches pulse-operated at the frequency of the alternating current output by the inverter of for example 50 Hz and connected in anti-parallel with external freewheeling diodes. The switches connected between the center points of the half-bridges and the other input lines are MOSFET semiconductor switches pulse-operated at a higher frequency of, for example, 16 kHz to form a sine-shaped output current of the inverter by means of pulse width modulation. These MOSFET semiconductor switches comprise inherent reverse connected diodes. According to the publication referenced here, such MOSFET semiconductor switches are not suitable on the other side of the half-bridges as their inherent diodes are too slow. For this known inverter, an EE (Euro Efficiency) at 2 kW nominal power of 99.2% is indicated. As the switches on the one side of the half-bridges are pulse-operated at an output frequency of the inverter, such as 50 Hz, it is not necessary to filter the current flowing through them. Thus, in the publication referenced here, it is proposed to connect the switches provided between the center points of the half-bridges and the one of the input lines only behind the respective inductance, directly between the respective output line and the one of the input lines, wherein, however, the respective freewheeling diode remains between the center point of the half-bridge and the one input line. A corresponding inverter is depicted in FIG. 14 of the publication. In this inverter, the IGBT semiconductor switches may be replaced by MOSFET semiconductor switches. In this way, an EE increased to 99.4% at 2 kW nominal power is achieved by this inverter, if the diodes remaining directly between the center points of the half-bridges and the one of the input lines are SiC diodes. The actual efficiency is told to be even higher, as the MOSFET semiconductor switches pulse-operated at the frequency of the alternating current output do not reach the temperature of 125° C. the simulation is based on. This efficiency is very high, and it is achieved with just a small number of parts. However, this known inverter—due to its topology—is generally only suitable for outputting pure active power. The statement of the publication referenced here that using FRED-FET semiconductor switches and accepting the disadvantages accompanied therewith including a reduced efficiency, the inverter would also be able to output reactive power, is not correct. Anyhow, in the publication referenced here, it is proposed to supplement the inverter of the already amended topology by two further SiC diodes each between one of the shifted semiconductor switches and the other input line, and to connect the center points of the thus formed additional half-bridges, each via an additional inductance, to the output lines for outputting reactive power. The topology of this inverter suitable for providing reactive power and depicted in FIG. 16 of this publication thus includes a total of four half-bridges each consisting of a semiconductor switch and a SiC diode, wherein the semiconductor switches in one pair of half-bridges are adjacent to the one and the semiconductor switches in the other pair of half-bridges are adjacent to the other input line, wherein an inductance is connected between the center point of each half-bridge and the respective output line, wherein the center points of one half-bridge per pair are connected to the same output line, and wherein the inductances each filtering towards one input line are coupled. Here, the use of SiC Schottky diodes is recommended. As the flowing current is filtered by two inductances in this topology, the high efficiency of the previously described inverter not suitable for providing reactive power, cannot be achieved.
It is further known that the two switches of each of two half-bridges of an inverter may be high frequency pulse-operated in diagonally arranged pairs to generate a sine-shaped output current of the inverter. One then refers to bipolar modulation. In bipolar modulation the output voltage high frequency jumps between the two input potentials of the inverter resulting in a high current ripple in the inductances. A further disadvantage is that the current freewheeling through the reverse connected diodes runs through the input side capacitances, resulting in additional losses due to the internal reactive power flow within the inverter. This known inverter is, however, generally suited for outputting reactive power.
There still is a need of an inverter with a high efficiency and the capability of outputting reactive power.